Multi-beam sources of the above mentioned kind can be used for a variety of applications, like lithography and microscopy systems. Some of the systems employing multi-beam sources use a single source generating one beam which is subsequently split into a plurality of beamlets. The charged particle sources used in such systems typically emit a charged particle beam with a defined opening angle, i.e. a diverging beam. The diverging beam often needs to be collimated, i.e. transformed into a homogeneous beam. In most applications a lens or lens assembly is used to refract the diverging beam emitted. Improvements of such multi-beam sources are currently the subject of intensive research activities all over the world.
A typical application of a multi-beam source is a multi-beam lithography system, e.g. in the semiconductor industry, for producing patterns on different substrate materials. Such apparatus usually comprise an illumination system with a particle source, generating a diverging beam of energetic particles and a lens system for forming said beam into a telecentric beam which illuminates different means for splitting the broad beam into a plurality of sub-beams. By means of an optical projection system the sub-beams are focused on a target which is typically some kind of substrate, e.g. a silicone wafer. Systems of that kind are disclosed in the US 2005/0161621 A1, US 2005/0211921 A1 and two documents by the applicant/assignee, namely the U.S. Pat. No. 6,989,546 B2 and the U.S. Pat. No. 6,768,125. However these systems have certain drawbacks since optical systems, regardless of whether they are light-optical or particle-optical systems, produce imaging aberrations and distortions. Therefore sub-beams projected on the target will get blurred and the spot size of the sub-beam is no longer well-defined, which results in a blurry pattern or image.
It is known and general practice for particle-optical imaging systems to use electro-static lenses in the form of two or three rotationally symmetrical annular electrodes, which are formed as a tube, ring or diaphragm, or rather arrangements of such elements in rows, where a beam passes through the middle of said annular electrodes which lie at least partly at different electric potentials. Lenses of this type always have a positive refractive power and are thus focusing lenses; furthermore without exception they have significant aberrations of the third (or higher) order which can only be slightly influenced by the shape of the lens geometry. A system employing such a lens setup is disclosed in the U.S. Pat. No. 5,801,388 by the applicant/assignee.
By using diverging lenses (negative refracting power) it is possible to ensure that the aberrations produced by the arrangement of combined focusing lenses and diverging lenses are to a great extent compensated by cancellation of the contributions to the third (or higher) order aberrations of the focusing and diverging lenses, the other coefficients of aberration are also maintained as small as possible. It is not possible by means of annular electrodes alone to achieve a lens of negative refracting power; on the contrary, it is necessary to use a plate or control grid electrode through which the beam passes. A system using the mask of a lithography apparatus to form diverging lenses in combination with annular electrodes located in front of and after the mask, respectively, is disclosed in the U.S. Pat. No. 6,326,632 B1 by the applicant/assignee.
As a result of the lens errors of focusing lenses, an illumination system which comprises focusing lenses and which produces a substantially telecentric ion beam has the characteristic that, for example, although the beams in the proximity of the axis are parallel to the optical axis, the beams remote from the axis are somewhat convergent or divergent. In the outer regions of the mask this would lead to image defects, especially if used in conjunction with a large reduction optical system (such as described in U.S. Pat. No. 6,768,125) where the angular errors at the object plane (aperture plate system) lead to significant landing angle errors at the substrate, or if used in conjunction with a parallel multi-column array, where the angular alignment of each beam in each column is very critical.
One solution for avoiding these shadow effects is the production of structure orifices which are inclined accordingly with respect to the axis; however this is extremely expensive from the technology point of view. An additional diverging lens disposed downstream of the focusing lens arrangement can render it possible to correct these errors and the excessive convergence of the beams remote from the axis can be compensated.
Such a solution is described in the article “Development of a multi-electron-beam source for sub-10 nm electron beam induced deposition”, J. Vac. Sci. Technol. B 23(6) (2005), pp. 2833-2839, by M. J. van Bruggen et. al. The authors therein describe a multi-beam source, where a broad beam of particles is split into 100 sub-beams with an aperture plate. The sub-beams are individually focused by a micro-lens array, creating a negative lens effect together with a subsequent electrode. Van Bruggen et. al. aim on compensating for both the third-order geometric and first-order chromatic aberration inherent in the system, however such a system can not provide for correction of the individual beams and aberrations due to insufficient illumination of the aperture plate.
The US 2004/0232349 A1 discloses a multi-beam source of the type the invention is related to. It comprises a particle source, a converging means and a lens array, placed between the source and the converging means to avoid the negative influences of the chromatic aberrations of the optical system. The lens array is substantially a plate with holes, interacting with annular electrodes placed before and/or after the lens array. In a variant of the invention as disclosed in the US 2004/0232349 A1, at least one deflector array with holes and deflectors aligned with the beamlets can be additionally included, which allows for asserting a deflecting effect proportional to the distance of a deflector from an optical axis of the respective beam. By virtue of such an arrangement, the beamlets can be controlled individually. However this solution has the significant drawback of requiring specifically shaped lens arrays, e.g. convex plates or stacks of multiple plates allotting inclined holes to account for the slope of the beamlets. Furthermore the lens arrays can scarcely be adapted to changing circumstances concerning the beamlets.
A comparable approach is described in the U.S. Pat. No. 7,084,411 B2 by the applicant/assignee, disclosing a pattern definition device for use in a particle-exposure apparatus. In said device a beam of energetic charged particles is patterned by a system of pattern definition means of substantially plate-like shape, each comprising a plurality of apertures, into a plurality of sub-beams. In order to correct for the individual aberrations that may be present in a particle-exposure apparatus, for each aperture at least two deflecting electrodes are provided for correcting the path of the sub-beam. The electrodes can be controlled individually or in groups.
In the WO 2006/084298 by the applicant/assignee, a solution for the above mentioned imaging aberrations and distortions in a charged particle exposure apparatus is proposed. The solution is applicable for instance in the IMS-concept PML2 (short for “Projection Mask-Less Lithography”) as described in the U.S. Pat. No. 6,768,125 by the applicant/assignee, in which a multi-beam direct write concept using a programmable aperture plate system for structuring an electron beam is disclosed. The WO 2006/084298 describes the provision of a diverging lens that is able to compensate for aberration errors of higher rank than third order and/or distortions, or to correct specific aberration coefficients, or to correct for misalignment. The lens is realized as a plate electrode means with a plurality of apertures, comprising a composite electrode composed of a number of partial electrodes, being adapted to be applied different electrostatic potentials. This plate electrode means realizes an electrostatic zone plate (EZP), which provides a simple and yet efficient means to implement a diverging lens and/or specific compensation for the imaging problems discussed above.